The perplexing irony of Barack Obama's presidency is that even as conservatives attack him as a crazed socialist, many on the left are frustrated with what they see as the president's accommodationist backtracking from campaign promises.

"If there is an interest group completely happy with Obama, they've done a great job of keeping quiet about it," said Jim Kessler, vice president of the centrist group Third Way.

The difference between the two sides is that the left's complaints are more, as they liked to say in the days of George W. Bush, reality-based. Obama has done things -- or, more often, failed to do things -- that have understandably disappointed various constituencies.

The president remains overwhelmingly popular with liberal Democrats. His problem, such as it is, is with what one party strategist called the "activist infrastructure."

To some extent, this is inherent in the nature of the job. No president can govern as purely as his most ideological supporters would like. The art of the possible requires trade-offs certain to rankle those who elevate the importance of one particular issue.

Some of the dissatisfaction, though, is unique to Obama. Bill Clinton faced his share of grumbling from the base: Remember the North American Free Trade Agreement and welfare reform? But Clinton ran as a different kind of Democrat, so party interest groups were at least warned.

Obama ran as, well, Obama -- a relatively unknown but charismatic vessel into which Democrats of any ideological stripe could pour their hopes. Disappointment was inevitable. No flesh-and-blood president could live up to the imagined heights of candidate Obama. If the swooning left had read Obama's policy manifesto, "The Audacity of Hope," it would have gotten a peek at Obama's style as president, elevating the achievable over the perfect.

The offshore-drilling decision perfectly illustrates Obamaism's pragmatic progressivism. Substantively, the president is attracted to split-the-difference solutions: allow drilling here, block it there. Politically, he is a horse-trader, not a line-drawer.

Given his supporters' "extravagant unrealism," says the Brookings Institution's William Galston, "there was no way he could fulfill all those promises -- not in his first year, not in his first term, not ever." Obama's decision to put all his chips on health care guaranteed that those with competing priorities would be frustrated. The unexpected length of the health-care fight only aggravated that reaction.

Galston points to another factor underlying unhappiness among Democratic Party constituencies: the "asymmetrical polarization" of the political parties. Unlike the GOP, which has consolidated its conservatism, the Democratic Party is ideologically diverse. The Democratic base lacks an ideological majority. Some of the party's core voters are destined to be disappointed some of the time.

Such unhappiness may drain energy and money from campaigns, but it's hard to imagine a primary challenge to Obama similar to Ted Kennedy's effort to oust President Jimmy Carter in 1980. After all, Obama managed to secure passage of health-care reform, albeit without the vaunted public option.

Then again, Obama faces the worst of three worlds. Conservatives see him as the reincarnation of Karl Marx. Liberals are frustrated by what they perceive as one sellout or another. And independents, disgusted by partisan bickering, worried about the economy and nervous about health reform, don't perceive any moderation.